1. FIELD OF THE INVENTION
The present invention relates to an improved gas turbine and a shroud for such a gas turbine, and more particularly, to a shroud for a gas turbine which shroud is made of a heat-resistant iron-base alloy in which grains are refined so as to improve ductility and thermal fatigue resistance, to a gas turbine using the shroud and to a method of producing the shroud.
2. DESCRIPTION OF THE RELATED ART
A shroud for a gas turbine is of a segment-shaped type in which the individual segments are mechanically coupled to each other. Generally, the segments are made of a heat-resistant iron-base alloy and are produced by precision casting. The members of the shroud are subjected during the operation of the turbine to rapid heating and cooling which are repeated as the gas turbine is started and stopped. The thermal stress thus occurring causes the surfaces of the shroud segments which are exposed to a combustion gas to be cracked. U.S. Pat. No. 4,615,658 discloses a shroud made of SUS 310 steel or an improved alloy containing a particular component. However, neither description nor suggestion regarding the macrostructure and grain size of the material for the shroud is given in this publication.
As stated above, the shroud for the gas turbine is produced by precision casting. However, in the prior art the grain size of the casting was large with the result that cracks occurred by the repetition of heating and cooling, that is, there had been such a problem that a shroud is poor in thermal fatigue resistance.